1. Field of the Invention
The present invention relates to a circuit for protecting transmission lines against transients, such as those caused by Electrostatic Discharge (ESD); more particularly, the invention relates to a semiconductor device having low capacitance and low leakage and which is useful for the protection of high speed data transmission lines which typically operating at voltages below 5V.
2. Description of the Related Art
With the increase in performance and component density of digital electronic circuits, which is due in part to improved sub-micron masking processes, there is a corresponding need to reduce power supply voltages to these circuits in order to reduce the internal power dissipation of the circuitry. A further advantage of the lower voltage power supply and corresponding low power consumption is the possibility of operating these circuits at higher clock speeds a heretofore possible.
Conventional logic voltage levels have generally decreased from the typical 5V range to 1.5V or lower. It is predicted that this logic voltage level will further decrease to the sub-one volt level in the next few years. Similarly, it is anticipated that the masking process will decrease from 1.5 microns to less than a few hundredths of a micron in the next few years.
A disadvantage of this increased scale of integration is that semiconductor devices, in particular metal oxide semiconductor (MOS) transistors, are now more susceptible to damage by over-voltage transient pulses. These pulses are generally caused by electrostatic discharge pulses or positive or negative voltage spikes generated during switching or just caused by the inductive effect of a transmission line impedance. A further cause of damage is as a result of accidental connection to the wrong voltage supply levels as, for example, by connecting a 5V supply voltage to a 2.5V or a 1.5V input on a device.
Many solutions to this problem have been proposed, such as the use of a single P-N (transient voltage suppressor) TVS/Zener device or clamp. However, these devices are ineffective in these low voltage applications since it is difficult to achieve low leakage levels at supply voltages below 5V. Another solution is to use steering diode line termination circuitry. However, these are effective to minimize the overshoot and undershoot switching events that could be absorbed in part by the power supply, but under an extremely fast transient event, as in an ESD, the circuitry could be damaged.
Currently some semiconductor component manufacturers, like SEMTECH Corp, offer very low leakage TVS/Zener devices, such as their part No. SLVU2.8, which is capable of handling ESD and transient protection levels intended for 3.3V systems. These devices are based on Low-voltage Punch-Through concept as described in U.S. Pat. No. 6,015,999.
A disadvantage of the above described devices is that they must be coupled in series with a rectifier diode in order to achieve capacitance levels in the order of 10 pF or below, so as to exhibit a low insertion loss.
These low capacitance values are necessary in order that the protection devices can be used to protect against ESD and transient events on high speed transmission data line ports, such as 10/100/1000 Ethernet, USB 1.1 and 2.0 and FireWire-IEEE 1394 or similar.
Other device solutions are described in U.S. Pat. Nos. 6,442,008; 6,163,446; 6,492,859; 6,268,639; 6,320,735; 6,015,999 and 6,268,990. However, for various reasons these devices do not exhibit the low leakage and low capacitance required for low voltage applications.
Accordingly there is a need for a protection device that exhibits low capacitance, low leakage at low voltages and that can preferably be used to protect high speed transmission lines.